D¶

DAD¶

Disciplined Agile Delivery. A process decision framework by Scott Ambler and Mark Lines. It combines practices from Scrum, XP, Lean and agile modelling, and helps teams choose how to work across the whole delivery lifecycle, including at scale.

Daily Meeting¶

A short meeting held at the same time each day, at which the team coordinates its work. Also called the daily stand-up.

Daily Scrum¶

Scrum’s daily stand-up: a meeting of 15 minutes or less in which the team reviews progress towards the sprint goal and plans the next day.

Dark Fibre¶

Optical fibre that is installed but not lit by its owner’s equipment. Customers lease it and attach their own equipment, so they control the capacity and technology rather than buying a bandwidth service.

DASS1¶

Digital Access Signalling System 1. An early BT protocol for ISDN access in the UK, replaced by DASS2 and long obsolete.

DASS2¶

Digital Access Signalling System 2. BT’s signalling protocol for ISDN primary rate lines (2 Mbit/s, 30 channels), designed before Q.931 was finished. Standard EuroISDN (Q.931) access has replaced it.

Data modelling¶

Analysing the data a business uses: identifying the entities, their attributes and the relationships between them.

Data Plane¶

The part of a network device that forwards packets, using tables built by the control plane. Also called the forwarding plane or User Plane.

See also: Control Plane, Management Plane.

Database Lead¶

On projects with large or critical databases, the person responsible for the design and construction of the database parts of the system.

Data Warehouse¶

A central store of current and historical data, integrated from several operational systems and organised for reporting and analysis rather than for transactions.

Data Warehouse

Data Warehouse.

Data Warehouse.

DCE¶

Data Circuit-terminating Equipment. The device that connects a terminal or computer (the DTE) to a transmission line, such as a modem.

DCE

DCE.

DDK¶

Driver Development Kit. The tools, libraries, headers and documentation needed to write device drivers for an operating system.

DDOS¶

Distributed Denial of Service. An attack that floods a target with traffic from many sources at once, often a botnet, to make it unavailable to legitimate users. Because the traffic comes from everywhere, blocking one source does not stop it.

DDOS

DDOS.

Decomposition Estimation¶

Estimating each part of a system after breaking it down. See Bottom-Up Estimation.

Defect¶

A flaw in a system, process or artefact that makes it fail to meet its specification, a standard or a reasonable expectation.

See also: Bug, Defect Report, Defect Management.

Defect Management¶

Recording, prioritising, tracking and closing defects so that none are lost.

Defect Report¶

A record asking for an artefact to be changed because it does not conform to a requirement or standard, or because it fails.

Definition of Done¶

DoD. A list, agreed by the team, of what must be true before a piece of work, such as a user story, counts as finished. It usually includes tests passing, review and documentation.

Definition of Ready¶

DoR. Criteria a backlog item must meet before the team takes it into an iteration, such as clear acceptance criteria and no blocking dependencies. Many teams base it on INVEST.

Deliverable¶

An artefact, or set of artefacts, that a project or piece of work hands over as its output.

Delphi¶

Embarcadero’s (originally Borland’s) development environment for Object Pascal, an object-oriented descendant of Pascal. The language itself is often called Delphi too.

Deployment¶

Installing a system and making it ready for use, including any integration and configuration for the particular site.

Deployment Lead¶

On projects with complex deployment needs, the person who plans and oversees deployment, and often operations and maintenance.

Deployment Plan¶

A plan for releasing a system, covering the target platforms, the configuration to release and the steps to follow.

See also: Deployment, Release Management.

DER¶

Distinguished Encoding Rules. A restricted form of BER that allows exactly one encoding for each ASN.1 value. It is used where encodings must be unique, above all for data that is signed, such as X.509 certificates.

DES¶

Data Encryption Standard. A symmetric block cipher from IBM, published as a US federal standard in 1977, with a 64-bit block and a 56-bit key. The short key made it breakable by brute force by the late 1990s, and AES replaced it.

DES

DES.

Design¶

Software design: working out the structure of a system and the models and plans for implementing its requirements.

Design Lead¶

The person responsible for a system’s architecture and for overseeing design work.

Design Pattern¶

A general, reusable solution to a problem that keeps coming up in software design. A pattern is a description to adapt, not finished code. The Gang of Four book catalogued the best-known object-oriented patterns.

Design to Tools¶

Deciding what goes into a system by what existing tools support directly, rather than by what users need.

Desk Check¶

An informal review in which the author asks one or more colleagues to read an artefact and look for defects.

Detailed Design¶

Design work below the architecture, covering parts of both high-level and low-level design.

Detailed Schedule¶

A fine-grained schedule of tasks, effort, people and dependencies, usually built bottom-up from a work plan. It often covers only the next few weeks and rolls forward.

Developer¶

A software engineer, especially when doing design and construction.

Developer Integration Test¶

DIT. An integration test that a developer runs in the system test environment after a project build, before releasing new functionality.

Development Environment¶

The hardware, software and tools in which developers build and test software.

Development process¶

The set of activities a team follows to develop software.

DevOps¶

A way of working that brings development and operations together, sharing responsibility for delivering and running software. It relies on automation of builds, tests, deployment and monitoring.

DHCP¶

Dynamic Host Configuration Protocol. The protocol a device uses to get an IP address and other network settings, such as the gateway and DNS servers, from a server when it joins a network.

Diagram¶

A graphical representation of a system, process or other information.

DLE¶

Digital Local Exchange. A digital telephone exchange that connects subscribers’ lines, either directly or through remote concentrators. In the UK these were System X and AXE 10 exchanges.

DLE

DLE.

DLE.

DLP¶

Data Link Protocol. Any protocol at the data link layer (layer 2) of the OSI model, such as Ethernet, Token Ring, FDDI or PPP. DLP also commonly stands for data loss prevention.

DMZ¶

Demilitarised Zone. A network segment that sits between the internet and an organisation’s internal network, holding the services that outsiders need to reach, such as web and mail servers. Firewalls on both sides limit what each zone can reach.

DMZ

DMZ.

DN¶

Directory Number. A telephone number: the digits that identify a subscriber’s line or device and route calls to it.

DNB¶

A Dun & Bradstreet business file: a company’s credit and identity record from Dun & Bradstreet.

DNS¶

Domain Name System. The internet’s distributed, hierarchical naming system. Its main job is to translate names such as example.com into IP addresses, but it holds many other kinds of record.

See also: Domain Name, FQDN, DNS - A, DNS Spoofing.

DNS - A¶

Address record. A DNS record that maps a name to an IPv4 address.

DNS - AAAA¶

A DNS record that maps a name to an IPv6 address.

DNS - CNAME¶

Canonical name record. A DNS record that makes one name an alias for another; the lookup carries on with the target name.

DNS - DNAME¶

A DNS record that makes a name, and every name under it, an alias for another domain. A CNAME aliases only the exact name.

DNS - SRV¶

Service record. A DNS record that gives the host and port for a named service, such as SIP or XMPP, together with priority and weight.

DNS - TXT¶

A DNS record that holds arbitrary text. It now mostly carries machine-readable data, such as SPF, DKIM and DMARC policies for email and domain ownership proofs.

DNS Spoofing¶

An attack that puts false records into a DNS resolver’s cache, so that it sends users to the wrong address, usually one the attacker controls. Also called DNS cache poisoning.

DoD¶

See Definition of Done.

DoR¶

See Definition of Ready.

Docker¶

A platform for building and running software in containers: packages that hold an application and its dependencies and run in isolation on a shared operating system kernel.

Document Collaboration¶

Tools that let several people work on the same document at once, from different places, to reach one final version. Wikis, Confluence and SharePoint are examples.

Domain Name¶

A name registered in the DNS, such as example.co.uk, that identifies an area of administrative control on the internet. Names form a hierarchy under the unnamed root: top-level domains, then second-level domains, then subdomains.

Domain Name

Domain Name.

Domain Name.

Dot1Q¶

IEEE 802.1Q. The standard for VLANs on Ethernet. It adds a 4-byte tag to each frame with a 12-bit VLAN identifier and a 3-bit priority (802.1p).

Dot1Q trunking¶

Carrying several VLANs over one link between switches. Each frame is tagged with its VLAN ID as it enters the trunk. The switch at the other end uses the tag to deliver it to ports in the same VLAN.

Dot1Q trunking

Dot1Q trunking.

Dot1Q trunking

Dot1Q trunking.

mermaid diagram

Dot1Q trunking.

Two VLANs sharing an 802.1Q trunk.

Downstream¶

In a project, the later activities and artefacts, such as coding, testing and deployment. In networking, the direction towards the customer.

DP¶

Distribution Point. In a UK access network, the point near the customer, often on a telegraph pole, where the drop wire to the premises joins the cable from the exchange. In fibre networks, the point where a fibre splits towards several premises.

DP.

DPI¶

Deep Packet Inspection. Examining the contents of packets, beyond the IP and transport headers, to classify, block, shape or log traffic. Firewalls, intrusion prevention and traffic management use it.

See also: Firewall, IPS, Packet sniffing.

DPNSS¶

Digital Private Network Signalling System. A BT-defined protocol for linking PBXs over digital private circuits, with a common set of features across vendors. QSIG is the international equivalent.

DQ¶

Data Quality. How accurate, complete, consistent and timely data is for its purpose.

Draft¶

An artefact under version control but not yet under change control.

Driver¶

Software that lets an operating system and applications use a particular device, hiding the details of the hardware behind a standard interface.

DSCP¶

Differentiated Services Code Point. A 6-bit field in the IP header that marks a packet’s traffic class, so routers can give it the right treatment under DiffServ quality of service.

DSDM¶

Dynamic Systems Development Method. An agile framework from the 1990s, built on rapid application development, that fixes time and cost and varies scope. It is now called the DSDM agile project framework.

DSL¶

Digital Subscriber Line. A family of technologies that carry digital data over copper telephone lines, using frequencies above voice. ADSL and VDSL are the most common.

DSL

DSL.

DSLAM¶

Digital Subscriber Line Access Multiplexer. The equipment in an exchange or street cabinet that terminates many customers’ DSL lines and combines their traffic onto one high-speed link.

DSLAM

DSLAM.

DTE¶

Data Terminal Equipment. The device at the end of a data link that is the source or destination of data, such as a computer or terminal. It connects to the line through a DCE.

DTE

mermaid diagram

DTE.

DTEs connected through DCEs.

DTLS¶

Datagram Transport Layer Security. A version of TLS for datagram transports such as UDP, giving the same confidentiality, integrity and authentication. It is used by CoAP and LwM2M in IoT and by WebRTC. The current version is DTLS 1.3, in RFC 9147.

DTMF¶

Dual-Tone Multi-Frequency. Telephone signalling in which each key sends two tones at once, one from a low group and one from a high group. It is standardised as ITU-T Q.23, and BT calls it MF4.

DTMF

DTMF.

DTT¶

Digital Terrestrial Television. Television broadcast digitally from ground-based transmitters, such as Freeview in the UK. It fits several channels into the spectrum one analogue channel used.

DTV¶

Digital Television. Television sent as digital signals rather than analogue, by terrestrial, satellite, cable or IP networks.

Duplex¶

A link that can carry communication in both directions. In full duplex both sides can send at once; in half duplex they take turns. A simplex link carries one direction only.

Duplex

Duplex.

Duplex

Duplex.

Duplex

Duplex.

mermaid diagram

Duplex.

Simplex, half duplex and full duplex.

See also: Full duplex, Half Duplex, Simplex.

DVSR¶

Dynamically Verified Static Routing. A Redback (later Ericsson) routing feature: a static route that the router adds to its table only while regular pings show the next hop is reachable.